OK, I figured it out: I had to download Power PC Enabler 1.1.1 from Apple, and use the IIsi to copy it to the System Folder on the new drive where I previously installed 7.5.0. Then I was able to boot the Power Mac 7100 from the drive. Boy, this machine is loud.
List of system enablers required for various Macs: http://support.apple.com/kb/TA28948
Apple download page to get the system enablers: http://www.info.apple.com/support/oldersoftwarelist.html#system
Good catch about the discrepancies, by the way. According to Apple, the 7100/80 actually requires 7.5.0 or newer, even though the 7100/66 works with 7.1.2.
List of system enablers required for various Macs: http://support.apple.com/kb/TA28948
Apple download page to get the system enablers: http://www.info.apple.com/support/oldersoftwarelist.html#system
7.5.x would be fine, but I have to install 7.5.0 first before I can apply any updates, right? Or maybe there's a direct path I missed. Anyway, it works now. Just need earplugs when working in the same room as this machine.
Good catch about the discrepancies, by the way. According to Apple, the 7100/80 actually requires 7.5.0 or newer, even though the 7100/66 works with 7.1.2.